Twin study of illness history variables in psychosis

Genetic factors are important in the aetiology of psychotic disorders, but it is unclear how far they influence aspects of illness history within psychoses.

To investigate the extent, and type of familial aggregation for a range of illness history variables in psychosis.

Two-hundred-and-twenty-four proband-wise twin pairs (106 monozygotic, 118 same-sex dizygotic), where probands had psychosis, were ascertained from the Maudsley Twin Register in London. We investigated the following illness history variables, rated using the OPCRIT checklist: age at onset; chronicity of course; mode of onset; psychotic/affective predominance; pre-morbid social adjustment; and the presence of a psychosocial precipitant. We used Mx statistical modelling software to analyse correlations of variables within pairs of monozygotic twins concordant for psychosis; and relationships between variables in probands and risk of psychosis in monozygotic and dizygotic co-twins.

There was a high monozygotic within-pair correlation for age at onset (intra-class correlation=0.9); moderate correlations for chronicity of course (polychoric correlation=0.4) and psychotic/affective predominance (polychoric correlation=0.5); and lower non-significant correlations for other variables. No variables consistently predicted risk of psychosis in co-twins.

Illness history variables in psychosis show a broad range of familial aggregation. It is likely that familial influences are predominantly modifying effects, independent of susceptibility factors for psychosis.